Improved event clone, improved event form's date/time, fixed recent changes for deletes

[Igal Koshevoy]

1. Improved the event clone feature so that the new event's time/date
fields have today's date, but the same time-of-day as the original
event. This makes it much easier to create clones of an event because
you typically just need to change the start date and the rest of the
fields will have the right values.

2. Improved the event date handling, changing the start date will now
update the end date if the start date is now after the end date, and
will highlight the end date to alert the user of the change. This makes
it much easier to reschedule an event to a future date, like when
cloning, because you'll typically only need to change the start date and
the end date will be updated for you.

3. Changing the start time will update the end time so that it's a
constant duration, I've added a highlight to alert the user of the change.

4. Fixed bug, the recent changes code now shows updates made to records
that had later been deleted without throwing exceptions or not showing
changes for that record.

Please reply if you run into any issues with this, otherwise I'll close
the related tickets in the near future.

-igal